Is it not actually rather better than that?
For instance, I approach the dragon shield set to 90%. I can pretty much laugh it's fire breath off (+4 saves and the equivalent of improved evasion from the shield)
Next it attacks, my shield still being at 90%, I get a +10 AC bonus just from the shield and again am pretty safe
Now my go. As a free action I adjust the shield to 50% get my attacks and then afterwards adjust the shield back to 90% again. Even if the dragon wises up and readies an action for when I move my shield I get full attacks against his standard attacks.
If I take a Disarm action to knock his wand out of his hand he gets no AOO because I have 50%+ cover
I'm genuinely puzzled by these things - they seem utterly broken. What's more it's a real headache thinking about GMing this. I hate house rules but the rules as written seem to force me to be much more creative and tactical with my monsters than I would like to be (I like most of them to simply charge and attack until killed). I don't want every fight to become a simple walkover or some 20 minute chess match of manouevring and counter-manouevring